When receiving radio signals, it is necessary to use an antenna that not only operates over the frequency range that the signals occupy, but that also matches the nature of the polarization of those signals. As is known to those skilled in the art, polarization describes the direction of the electrical field component of an electromagnetic (EM) wave, as it arrives at the receiving antenna. The electrical field component of an EM wave can be subdivided into a horizontal component and a vertical component.
If the electrical field component of the wave has only one subcomponent, either a horizontal component or a vertical component, then the wave is said to have linear polarization. If the wave has both subcomponents the signal is said to have elliptical polarization. If the horizontal and vertical components are equal in magnitude and differ in phase by 90°, the wave is said to be circularly polarized. Either type of polarization, linear or elliptical, can provide two orthogonal signals at the same frequency. For example, a linear polarized signal can either propagate with its polarization in the horizontal direction or the vertical direction; and a circularly polarized signal can either be right-handed or left-handed, depending on the direction the electrical field vector rotates.
An antenna that is simultaneously operable in both orthogonal polarizations is advantageous because using each orthogonal polarization to independently carry data may double the capacity of a communications channel. In addition to increasing the capacity of a communications channel, polarization of a radio signal can be used to maximize the strength of a received signal by matching the antenna to the incoming polarization. It can also be used to eliminate an unwanted signal by setting the receive antenna to be orthogonal to the unwanted signal.
Dual polarized antennas have been realized in several different fundamental antenna forms such as dipole type antennas, waveguide-type antennas, reflector-type or lens antennas and helical antennas. Helical antennas, in particular, are well suited for satellite applications because they have a relatively large bandwidth and since it is possible to stow them in a small volume. A helical antenna typically consists of a conducting wire wound in the form of a helix and mounted over a ground plane. The helical antenna can operate in either normal or axial mode. In axial mode, the helical antenna is a natural radiator of circularly polarized radiation and can be configured to provide both hands of operation. FIG. 1 illustrates an isometric view of a typical axial mode helical antenna 5.
A common form of dual-polarized helical antenna is a dual polarized single-wire helix antenna. FIG. 2 illustrates a side view of a typical dual polarized single-wire helix antenna. The antenna 10 is comprised of a single wire helix 12, a reflector or ground plane 14, a lower end coaxial feed 16 and a far end feed 18. When the antenna 10 is fed from the lower end 16 the polarization is defined by the handedness of the single-wire helix 12. When the antenna 10 is fed at the far end 18, the helix 12 radiates its own particular hand of polarization, but this is reversed when reflected by the ground plane 14.
The most significant operational constraint of the dual polarized single-wire helix antenna 10 is its size. The antenna 10 will only radiate circular polarization in the axial mode when its circumference is about one wavelength (λ). Furthermore, the ground plane 14 must be sufficiently large to support successful wave propagation on the single-wire helix 12, and this can typically be larger than a wavelength (λ) across.
Attempts to design dual polarized forms of helical antennas have failed generally because the coupling between the two structures destroys the performance of both, or introduces a very high degree of electrical coupling between the two antennas or antenna elements.